1. Field of the Invention
This invention relates to equipment for disclosing pressure, and particularly to sports equipment capable of detecting and providing a temporary indication of pressure, such as that which would result from an impact upon that equipment.
2. Description of the Background
It is a common cause of disagreement between tennis players, often vehemently expressed in professional tennis, as to whether a ball fell within or without the boundaries of the court or the service area. There are means of detecting a ball landing outside the service area, but they are not infallible.
There are a number of faults which cannot be detected, for instance:                a foot fault during service,        a ball landing on the service line, including the center service line, and        a ball landing in or out of play.        
There are, in addition, a number of other sports where disagreement can arise, for instance, whether a footballer (association, rugby union, rugby league or American) has or has not put a foot in touch when playing the ball. There is consequently a need for a means of providing visual confirmation as to whether or not a ball has landed on the demarcation lines of a court for racket sports, or the boundary of a sports field. There are other applications as well where a visual indication of pressure, whether from a sudden impact or from a gentler surface to surface contact, would be useful.
WO 94/28982 discloses a device comprising a sandwich of material that is able to adhere to the face of a golf club, and provides an indication of the position of the impact of the face upon a ball. The sandwich comprises an outer impact layer and an inner adhesive layer, between which is a layer of thermnochromic material, such as a liquid crystal material.
A similar device is disclosed in US-A-5805245, but has two or more layers of thermochromic material that change color in different temperature ranges.
DE-A-4100434 discloses a material for marking lines or boundaries on sports fields or courts. An impact is disclosed by means of a liquid crystal material, employing a variety of direct or indirect means, but no detail is given as to how these means can operate.